Various abbreviations that appear in the specification and/or in the drawing figures are defined as follows:
3GPP3rd Generation Partnership ProjectBBbase bandBSbase stationCORDICcoordinate rotation digital computerCPcyclic prefixDACdigital to analog converterDCdirect current (in this context: zero frequency)DFTdiscrete Fourier transformDFTSdiscrete Fourier transform spreadEUTRANevolved universal terrestrial radio access networkFIRfinite impulse responseIEEEinstitute of electrical and electronics engineersIDFTinverse discrete Fourier transformIQin-phase/quadratureLTElong term evolution (3.9G)MIMOmultiple input multiple outputMSmobile stationOFDMorthogonal frequency division multiplexingOFDMAorthogonal frequency division multiple accessPApower amplifierRFradio frequencyRXDFEreceiver digital front endTAtiming adjustmentTXDFEtransmitter digital front endWiMAXworldwide interoperability for microwave accessUMTSuniversal mobile telecommunications systemUTRANUMTS terrestrial radio access network
This invention is related to physical-layer wireless communications, and is described in the context of wireless OFDM communications such as OFDM/OFDMA based wireless communication systems. It is applicable to wireless standards 3GPP EUTRAN/LTE/3.9 G and to IEEE 802.16d/e/WiMAX, though not necessarily limited thereto and can be extended to any OFDM based wireless protocol.
In a cellular OFDMA-based wireless communication system, during uplink transmission, the transmitted signals from mobile stations (MSs) have to reach the base station (BS) approximately at the same time. The time of arrival differences should be smaller than the length of the cyclic prefix CP for demodulation. However, the distances between the various MSs and the BS may vary considerably, possibly by several kilometers depending on cell size. This can result in large variations in the signal propagation delays from the MSs to the BS. To equalize this propagation delay variation, the BS periodically sends control messages, commonly termed timing adjustment or timing advance (TA) messages, to the MSs to adjust their transmission times. The MSs then individually advance or delay their transmitted signals according to the TA value in the TA message. Since the BS sends the TA messages, the individual timing adjustments are such that the transmitted signals from the MSs reach the BS at the same time.
FIG. 2 is a prior art block diagram showing a conventional OFDMA-based transmit chain and receive chain. Each of the BS and the MSs include both chains, but for clarity the transmit chain is described with reference to an MS and the receive chain is described with reference to the BS since in the description of the invention below it is the MS that sends its timing adjusted signal to the BS. The OFDMA transmitter 20A in the MS takes a data bit stream as input and generates the base-band transmit signal, which is up-sampled and filtered in the digital front end (TXDFE) 22A. The signal is converted into an analog waveform and up-converted to pass-band in the RF transmitter stage 24A and transmitted through the transmit antennas 26A (only one shown). The RF stage of the receiver 24B in the BS receives the transmitted signal from the antenna(s) 26B, down-converts it in base-band and produces a digital signal stream. The receiver digital front end (RXDFE) 22B processes this signal by down-sampling and filtering (and performing other functionalities not relevant to these teachings), and the OFDMA receiver 20B demodulates the sent information-bearing symbols, producing estimates of the sent data bits. Even though FIG. 2 shows only one transmitter for the MS, in OFDMA systems there may be a number of MSs transmitting to the BS receiver at the same time.
Depending on the distance between these various MSs and the BS receiving their transmissions, the time it takes for the transmitted signals to propagate to the BS may vary considerably. In order for the BS to demodulate the transmitted signals from the MSs, these signals must arrive at the BS within a certain time window. In OFDMA systems, this time window is generally determined by the duration of the CP and the delay spread of the channel. To ensure that these signals arrive within that window, the MSs further away from the BS should transmit earlier, while the MSs closer to the BS should delay their transmitted signals. This is why the BS periodically sends TA control messages to the MSs as noted above, so that the multiple MSs could adjust their transmit timing in a coordinated fashion.
Exactly how the MSs impose the TA delay or advance in an OFDM system is not a new problem. In the prior art the TA functionality was implemented in the time domain by up-sampling and inserting/deleting samples to or from the up-sampled signal. In the diagram of FIG. 2, this TA advance or delay is therefore imposed at the transmit digital front end 22A. Specifically, a polyphase upsampling finite impulse response FIR filter for TA is included within the TXDFE 22A, or in other instances it was disposed in the OFDMA transmitter 20A. The coefficients of this FIR filter are periodically connected to the multipliers by commutator switches. The TA is then done by advancing or delaying the commutators with control logic. This prior art solution is not specific to OFDMA, and has been used in various devices adapted for other wireless protocols (e.g., global system for mobile communications GSM and wideband code division multiple access WCDMA). It is also used in some EUTRAN/LTE/3.9 G uplink transmitters. However, this is seen as somewhat inefficient as the upsampling factor needs to be increased by an amount sufficient to impose the TA advance/delay with sufficient precision. This higher upsampling factor increases the sampling frequency of the signal, and greatly increases processing load and power consumption. Also, a simple polyphase filer to implement this prior art approach requires additional control signals to the commutator switches to impose the TA functionality by holding or skipping a number of commutator switch positions.
Another prior art approach by which to impose the TA advance or delay is to insert the CP after up-sampling, but to insert a shorter or longer CP in order to advance or delay the transmitted signal by the amount of the TA given by the BS.
What is needed in the art is a way to implement for a transmitted signal with arbitrarily precise timing advances/delays based on the TA messages received from a base station. Preferably, such precision is at less computational overhead than the polyphase filter implementation detailed above for the prior art.